The invention relates to a device for measuring the drop time of control rods into the core of a nuclear reactor and in particular into the core of a pressurized-water nuclear reactor.
Nuclear reactors, such as pressurized-water nuclear reactors, comprise a vessel closed at its upper part by a closure head, containing the reactor core which consists of fuel assemblies, of prismatic overall shape, which are placed in adjacent arrangements, with their axes vertical.
Each of the fuel assemblies comprises a set of vertical guide tubes in each of which a neutron absorber control rod neutron absorber, for controlling the reactivity of the nuclear reactor core, can move.
To adjust the reactivity of the core and to control the nuclear reactor, control rods are used which are moved in a certain number of assemblies distributed over the cross section of the core.
For example, for a nuclear reactor whose core comprises two hundred and five fuel assemblies, seventy-three control rods, which are moved in seventy-three core fuel assemblies, are used.
Each of the control rods comprises a control rod cluster consisting of neutron absorbers which are connected, in mutually parallel arrangements, to a fastening and suspension device called a spider assembly which is connected to a drive shaft made in splined form, so as to move the control rods step by step over the height of the nuclear reactor core, in order to adjust the reactivity.
The control rods are moved over the height of the core by mechanisms which are fastened to vertical tubular enclosures placed above the vessel closure head. The drive shafts, which are moved in a coaxial arrangement inside the tubular enclosures, engage via their splines with latch arms of the mechanisms allowing the control rod to be moved.
The nuclear reactor control rods also have a safety function, since they must allow emergency shutdowns of the nuclear reactor to take place, by the control rods dropping back into the position of maximum insertion in the nuclear reactor core.
To do this, the control rod mechanism releases the drive shaft so that the rod and the drive shaft can drop back into the nuclear reactor core under the effect of gravity.
In order to check the effectiveness of the nuclear reactor emergency shutdown device, it is necessary to determine and check the drop time of the control rods into the nuclear reactor core, this measurement and checking operation having to be carried out periodically, for example after the nuclear reactor has been shut down for refuelling and maintenance, before restarting the nuclear reactor.
To measure the drop time of the control rods of a nuclear reactor, the instrumentation for measuring the position of the control rods is used, which instrumentation in particular comprises measurement windings surrounding each of the tubular enclosures placed vertically above the vessel closure head, to which enclosures are fastened the mechanisms for moving the control rod clusters via the drive shaft which is mounted so as to move inside the tubular enclosure. The windings for measuring the positions of the control rods comprise a primary winding placed approximately over the entire axial length of the tubular enclosure and a plurality of secondary windings spaced apart in the axial direction of the tubular enclosure. The presence or absence of the drive shaft in a region of the tubular enclosure within a secondary winding results in different values of the voltage induced in the secondary winding by the primary winding.
It is thus possible to determine the position of the drive shaft inside the tubular enclosure and, from this position, to determine the position of the control rod cluster inside the nuclear reactor core.
When the control rod clusters drop back down, the movement of the drive shaft in the axial direction inside the tubular enclosure produces an induced voltage in the primary winding, this induced voltage varying according to the rate of movement of the drive shaft and to the length of the section of drive shaft within the primary winding. By recording the voltage induced in the primary winding, during the drop of the control rod from the high position, that is to say from the position in which the control rod cluster is fully extracted, it is possible to determine the drop time of the control rod.
To make the measurements on the various control rods of the nuclear reactor, temporary connections have to be made to each of the cables connecting a device for measuring position of a control rod to a unit for collecting the measurement signals, in order to pick up the voltage at the terminals of the primary winding.
The measurements are carried out in succession on control rod subgroups distributed within the nuclear reactor core.
For example, for a nuclear reactor comprising seventy-three control rods, the measurements are carried out in succession for the subgroups of four control rod which are placed in four parts of the core, called quadrants separated from one another by two planes of symmetry at 90xc2x0 to each other passing through the axis of the nuclear reactor core. Thus, nineteen successive measurements are carried out in order to determine all the drop times of the seventy-three control rods of the nuclear reactor.
The time needed to carry out the test is therefore quite long, given the fact that it is necessary to make an individual connection to the test device for each of the measurements and to return to the initial operating configuration of the device between two successive measurements.
The drop time measurements are carried out before starting up the nuclear reactor and lie on the critical path for shutting down the nuclear power unit, thereby representing a significant cost because of the increase in the time needed to shut down the nuclear reactor power unit.
In addition, the temporary connection operations create a risk of failure of the connection system on a safety-rated item of equipment.
Hitherto, no measurement device making it possible to carry out the measurement on the set of control rods of the nuclear reactor and to avoid connection operations prior to each of the measurements was known.
The object of the invention is therefore to provide a device for measuring the drop time of control rods into the core of a nuclear reactor comprising a vessel closed at its upper part by a closure head and containing the reactor core consisting of fuel assemblies in vertical adjacent arrangements and comprising vertical guide tubes for neutron absorbers of a plurality of control rods each consisting of a neutron absorber rod cluster and a drive shaft connected, at one of its ends, to the control rod cluster, a plurality of tubular enclosures placed vertically above the vessel closure head, each tubular enclosure comprising a mechanism for moving a control rod via its vertical drive shaft which can move in the axial direction of the tubular enclosure and for releasing the control rod in order to ensure that it drops into the position of maximum insertion in the nuclear reactor core, at least one electrical measurement winding surrounding each tubular enclosure, substantially over its entire axial length and connected to a cable for picking up an electrical voltage at the terminals of the winding, the control rods being grouped together in subassemblies and each electrical winding for each of the control rods of a subassembly being connected via its cable to a unit for collecting the measurement signals, this device making it possible to carry out the measurement on all of the control rods of the nuclear reactor at once and without having to carry out a prior operation of connecting the measurement cable.
For this purpose, the measurement device also includes:
a real-time measurement-signal acquisition module;
means for connecting the pick-up cable for each of the electrical measurement windings to the acquisition module in order to receive measurement signals corresponding to the voltage at the terminals of each of the electrical windings; and
means for using the voltage signals, which means are connected to the acquisition module in order to measure the drop time of the nuclear reactor control rods.